The onset of a cooler climate 4 million years ago — rather than tectonics — helped carve deep canyons in the Andes, geologists report in the Aug. 16 Science.

The researchers examined one of the many 1.5- to 2.5-kilometer-deep canyons that cut into the northeastern edge of the Andean Plateau in southern Peru. By dating when rocks at various depths within the chasm were exposed to the surface, the team determined that rapid erosion created the canyon.